The Internet and Internet Protocol (IP) networks use Internet Protocol version 4 (IPv4) or Internet Protocol version 6 (IPv6). These networks are networks that employ the IP for layer 3, that is, the network layer, of the Open Systems Interconnection (OSI) reference model. Ethernet (registered trademark) is a protocol that handles layer 2, that is, the data link layer, of the OSI reference model.
In general, for the aforementioned networks, there exists a technique that allows a certain network to be overlaid with a different network. For example, an IPv4-based service is implemented over an IPv6 network by using the IPv4 over IPv6 technique. Also, for example, implementation of an Ethernet-based service over an IPv6 network is also conceivable.
Such services are advantageous in that IPv4-based or Ethernet-based services are implemented in an available range of an IPv6. The absence of such an overlay technique results in construction of new infrastructure without utilizing an existing one, which is uneconomical.
The following is an example of a network system that uses the aforementioned overlay technique. For example, multiple IPv6 networks which are stub networks are connected to an IPv6 network which is a backbone network. Further, each stub network is overlaid with a corresponding IPv4 network.
In the case where an IPv4 packet is transferred from a certain IPv4 network to another IPv4 network in such a network system, an encapsulation-decapsulation device disposed at the boundary of the certain IPv4 network and the backbone IPv6 network encapsulates the IPv4 packet in an IPv6 packet and the resulting IPv6 packet is transferred within the backbone IPv6 network. The IPv6 packet is then received by another encapsulation-decapsulation device disposed at the boundary of the other IPv4 network and the backbone IPv6 network and the IPv4 packet extracted from the IPv6 packet is transferred to the other IPv4 network.
The related art is described, for example, in International Publication Pamphlet No. WO2001/099354.